1. Field of the Invention
The present invention relates to an apparatus for measuring a temperature of a high temperature liquid using an optical fiber and a method therefor and in particular to the apparatus and the method which allow a continuous measurement.
2. Description of Related Arts
A prior art method for measuring a temperature of molten metal in a furnace such as a converter processing a decarburization of molten iron, one of the steel smelting furnaces, adopts a sublance method or a manually operating thermocouple measurement method. In the sublance method, a thermometer element attached at a tip of a movable lance is inserted through a raw material charge hole on the furnace or other hole to perform the temperature measurement. Accordingly, the sublance method has disadvantages given below.
Firstly, a size of apparatus for measuring a temperature is enlarged in proportion to a size of the furnace. Secondly, replacement of a thermometer element is forcedly required in every measuring cycle because the thermometer element is of a single use disposable type, and further the replacement of the thermometer element is also required. Thirdly, intermittent measurement of the temperature forcedly carried out during operation because the consumption type thermocouple is forcedly replaced in every measurement cycle. And furthermore, frequent measurement is not allowed in view of an economic reason because of the disposition of such a thermometer element every measurement cycle. As a result, an actual observed temperature at the end of the process does not always agree with a target temperature, which often causes problems of generation of energy loss, increase of production cost, and degradation of productivity.
As a known technology of measuring a temperature in a furnace continuously, JP-A-91529/1986 (the term "JP-A-" referred to hereinafter signifies "Unexamined Japanese Patent Publication") discloses a method for measuring a temperature in a furnace using an optical fiber. In the disclosed method, however, the optical fiber is fixed at the tip of a nozzle so that the temperature measurement is carried out through a gas layer which is ejected from the tip of the nozzle. For this reason, this measuring method raises a serious problem on accuracy of the measured result. In addition, the tip of the optical fiber is exposed to an elevated temperature for a long period, which raises a problem of change of characteristics with passing time.
JP-A-203716/1988 discloses another method of continuous measurement of a molten steel. The method relates to a control method of making the molten steel by blowing in which the temperature of molten steel is continuously measured so as to agree during and at the end of blowing stage with the target value, and in which the composition at the end point is estimated from the change rate of the temperature increase. The patent specification describes "a method for measuring a temperature of a molten steel is applicable to using a radiation thermometer connecting an optical fiber, which thermometer immerses the optical fiber into the molten steel in a reaction vessel such as a converter through its bottom wall, side wall, or top cover". Nevertheless, in the specification no concrete description is given and no technological content is provided.
The inventors proposed an apparatus for measuring a temperature in the preceding Japanese Patent Application No. 78736/92 (JP-A-248960/93). The patent application described an apparatus wherein a tip of an optical fiber covered with a metallic tube is immersed into molten steel as a thermometer element, using the principle that a black body radiation condition is established at the tip of the optical fiber, and wherein the other end of the optical fiber is connected to a radiation thermometer. Also the patent application describes an apparatus for measuring a temperature in which an optical fiber feed means is installed to send out and also to retract to wind the optical fiber covered with a metallic tube and in which the tip of the optical fiber covered with a metallic tube is immersed into the molten steel for a short period only during the time for measuring the temperature thereof and is taken out from the molten steel immediately after completing the temperature measurement and in which the tip once used as the thermometer element is cut off to discard, and a new tip is fed at next measuring cycle.
The apparatus for measuring a temperature disclosed in Japanese Patent Application No. 78736/92 (JP-A-248960/93) is, however, a mere intermittent measuring apparatus, and the apparatus requires that the used tip be cut off every measurement cycle. As a result, there is a problem in that that type of apparatus fails to perform a continuous measurement for a long period.